Bounding information is used to limit the output to a specific geographical area. Unless you are also requesting a companion output file, you don't absolutely have to specify bounding information. However, if you don't, the output file will contain all of the airports in the FAA's NASR database.
Instead of the rectangular latitude/longitude coordinate method employed in most programs, this site uses circular bounding. You specify the three or four letter FAA/IATA identier of an airport at the center of your area of interest and the maximum number of points (including both airports and other turnpoints) your output file should contain. The system will find the specified number of airports closest to the center airport and include those in the output file. For example, if you want your output file to specify the 500 closest airports to Salt Lake City, you would specify SLC as the center airport ID and 500 as the maximum number of waypoints.
Alternatively, you can chose to specifiy a fixed radius (in nautical miles) from the center airport. In this case, the output will include all the airports within a specified radius.
Another way of specifying bounding information is to upload a turnpoint file. If you do so without giving explict bounding information, the output file contents will be limited to the list of airports and waypoints in the turnpoint file. This"implicit bounding" feature can be useful when you simply wish to update the airport information in the turnpoint file. It can also be used to convert an input file to one of the companion file formats, for example, from .CUP to .KML.
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